1. Field of the Invention
The present invention generally relates to a route optimization method and an agent apparatus and, more particularly, to a route optimization method and an agent apparatus by which an IP packet route is optimized in a communication system employing the Mobile IP protocol.
2. Description of the Related Art
Mobile IP, proposed by Internet Engineering Task Force (IETF), is known as a method for a mobile terminal having an IP address to maintain communication without terminating a session even when the mobile terminal moves across network domains.
FIG. 30 shows how a packet is transmitted according to the Mobile IP protocol of the related art.
Referring to FIG. 30, the mobile terminal 11 has a permanent IP address in a home domain (hereinafter, simply referred to as a home address). A home agent (HA) 13 is located in the home domain to which the mobile terminal 11 belongs. Numeral 41 indicates a correspondent terminal with which the mobile terminal 11 communicates. The correspondent terminal 41 may be a land unit or a mobile unit. An abbreviation CN/MT (correspondent node/mobile terminal) will also be used to denote the correspondent terminal 41. Numeral 201 indicates a foreign agent (FA) located in a destination domain of the mobile terminal 11 and has a care-of address.
A description will now be given of how a packet is transmitted according to Mobile IP.
After moving to a destination domain, the mobile terminal 11 acquires an address assigned to itself in the destination domain in the form of a care-of address. The mobile terminal 11 notifies the home agent 13 of the care-of address via the foreign agent 201. The home agent 13 registers the care-of address from the mobile terminal 11 in relation to the home address of the mobile terminal 11.
The correspondent terminal 41, requesting an IP packet to be sent to the mobile terminal 11, sends an IP packet destined for the home address of the mobile terminal 11. The IP packet arrives at the home agent 13 via an IP network 220. The home agent 13 encapsulates the IP packet based on information registered for the mobile terminal 11 and then forwards the encapsulated packet to the care-of address of the mobile terminal 11. The foreign agent 201 receiving the encapsulated IP packet decapsulates the packet and delivers the original IP packet to the mobile terminal 11 having the care-of address.
When the mobile terminal 11 is located in the home domain, the home agent 13 operates as a router so that the IP packet addressed to the mobile terminal 11 is routed to the mobile terminal 11.
Thus, even when the mobile terminal 11 moves across domains, the IP packet addressed to the mobile terminal 11 is ensured to arrive at the mobile terminal 11 via the home agent 13 and the foreign agent 201.
One problem with the base Mobile IP protocol is that, when the IP packet addressed to the mobile terminal 11 arrives at the mobile terminal 11 via the home agent 13, a packet may follow a path which is longer than an optimal path. When this triangular routing occurs, a relatively large transmission delay may be incurred. The transmission delay presents a serious problem when the data transmission is concerned with voice over IP datagrams, which are relatively less tolerant of delay.
An internet draft from the IETF (draft-ietf-mobileip-optim-08.txt) proposes Route Optimization to resolve the problem of transmission delay caused by triangular routing. Route Optimization uses a protocol for establishing a shortcut path from the correspondent terminal to the care-of address of the mobile terminal 11, bypassing the home agent 13. FIG. 31 shows Route Optimization applied to the related-art Mobile IP.
In the proposed Route Optimization, the correspondent terminal 41 acquires the care-of address of the mobile terminal 11 by periodically sending a Binding Request Message to the home agent 13 for the mobile terminal 11 and receives a Binding Update Message therefrom. The correspondent terminal 41 extracts the current care-of address of the mobile terminal 11 from the Binding Update Message periodically obtained. The correspondent terminal 41 encapsulates the IP packet destined for the mobile terminal 11 and transmits the encapsulated packet to the care-of address.
Thus, as shown in FIG. 31, the IP packet from the correspondent terminal 41 addressed to the mobile terminal 11 arrives at the mobile terminal 11, bypassing the home agent 13. This way, the problem with triangular routing is resolved.
One disadvantage with Route Optimization described above is that the correspondent terminal 41 must exchange messages with the home agent 13 for the mobile terminal 11 periodically, making it difficult to reduce power consumption in the correspondent terminal 41. This presents a problem especially when the correspondent terminal 41 is a battery-powered mobile unit.
In the related-art Route Optimization, the size of a header portion of the IP packet doubles due to encapsulation, thus preventing the bandwidth for transmission from the correspondent terminal 41 to the foreign agent or the home agent of the domain to which the correspondent terminal 41 belongs from being effectively used. Particularly, VoIP packets presents a serious problem since the size of a header portion is relatively large in relation to the data portion.